1. Field of the Invention
The present invention relates to a voltage selecting circuit for outputting a voltage corresponding to an input digital signal.
2. Description of the Related Art
In recent years, a liquid crystal television and a liquid crystal PC monitor have been rapidly spread. Also, in association with a higher function of a portable phone, the need for a liquid crystal display panel of a large scale and a high definition has been expanded. Under such background, the market of a driver for driving a liquid crystal display panel has been sharply grown, and the drop in the manufacturing cost of the liquid crystal display driver is desired more and more.
A digital/analog (D/A) converting circuit is built in the liquid crystal display driver. This D/A converting circuit is the circuit for converting an image data of a digital format into an analog gradation voltage that is applied to a pixel. Thus, this D/A converting circuit can be referred to as [Gradation Voltage Determining Circuit] for determining a gradation voltage corresponding to the image data.
FIG. 1 shows the configuration of a typical gradation voltage determining circuit 50. For example, this gradation voltage determining circuit 50 can output 64 gradation output voltages (gradation voltages) V0 to V63 based on a 6-bit digital image signal D0 to D5. Specifically, the gradation voltage determining circuit 50 has a gradation voltage generating circuit 51 and a gradation voltage selecting circuit 52. Reference voltages Vref0 to Vref9 are supplied to the gradation voltage generating circuit 51 from an external power source. This gradation voltage generating circuit 51 has a resistor array composed of 64 resistors R1 to R64. The input reference voltages Vref0 to Vref9 are suitably divided by the resistor array. Consequently, the gradation voltages V0 to V63 of 64 stages are generated.
On the other hand, the gradation voltage selecting circuit 52 receives the digital image signals D0 to D5 and the gradation voltages V0 to V63 and selects one gradation voltage from among the gradation voltages V0 to V63 based on the digital image signal. In short, the gradation voltage selecting circuit 52 carries out the role for decoding the digital image signal D0 to D5. Typically, a breakdown voltage of 12 to 18 volts or more is required for the liquid crystal display driver. The gradation voltage selecting circuit 52 serving as a decoder is composed of a large number of high breakdown voltage MOS transistors which have the matrix-shaped layout. One gradation voltage selected by the gradation voltage selecting circuit 52 is outputted from an output terminal OUT and applied to the pixel.
FIG. 2 shows an ideal relation (referred to as [V-T Characteristic]) between output voltage (gradation voltage) V and light transmittance T of a liquid crystal. As shown in FIG. 2, the ideal V-T characteristic is represented by a non-linear curve. By adjusting the reference voltages Vref0 to Vref9 supplied to the gradation voltage generating circuit 51, it is possible to compensate the output voltage and make the V-T characteristic approximate to the ideal shape.
As the conventional technique related to the liquid crystal display driver, a reference voltage switching circuit is disclosed in Japanese Laid Open Patent Application (JP-P2001-36407A). This reference voltage switching circuit has a digital data voltage decoding circuit corresponding to the gradation voltage selecting circuit 52. The decoding circuit is divided into a plurality of blocks 52-1 to 52-I, as shown in FIG. 1. Then, a well voltage of the MOS transistor included in each block is set to be different for each block. That is, a voltage applied to a back gate of the MOS transistor is different for each block.
Also, Japanese Laid Open Patent Application (JP-A-Heisei, 8-279564) discloses a voltage selector circuit corresponding to the gradation voltage selecting circuit 52. The voltage selector circuit is provided with a plurality of MIS transistors for outputting selection voltages, and is also divided into a plurality of blocks as shown in FIG. 1. Then, a channel length of the MIS transistor is designed to be different for each block. Specifically, the channel length of the MIS transistor to which a substrate bias effect is applied by selecting the middle selection voltage is designed to be shorter than the channel length of the MIS transistor to which the substrate bias effect is not applied by selecting the highest or lowest selection voltage.
This inventor paid attention to the following points. That is, a large number of high breakdown voltage MOS transistors that have an offset gate structure are used in the gradation voltage selecting circuit 52 shown in FIG. 1. The size of the high breakdown voltage MOS transistor is large, and the area of the gradation voltage selecting circuit 52 that requires the large number of high breakdown voltage MOS transistors becomes very large. This fact leads to the increase in the cost of the liquid crystal display driver. In particular, in the liquid crystal display for TV, the liquid crystal display driver that can display 1,000,000,000 colors is required in order to attain a larger scale screen size and the higher image quality display. For this reason, the gradation voltage selecting circuit 52 that can treat the output voltage of 1024 gradations (10 bits) is required. Thus, the increase in the circuit area caused by the increase in the number of elements becomes more severe. This results in the further increase in the cost of the liquid crystal display driver.